For those who were wondering these aren't just for the UK. They apply to most of Europe and other parts of the world as well.
For example, these numbers are also used (sometimes) in Brazil. They are used on products that are made here - not just imports, however - I would recommend having these numbers on your cellphone to consult if necessary, especially if you are shopping in stores with imports even if you are in the US.
For those wondering about different E numbers not on Melissa`s list and whether they can/can't wikipedia has a complete list you can use as reference: